1. Field of the Invention
The present invention relates to a method of controlling a video camera with autofocus function and more particularly to a method of controlling a drive of a lens for focusing (image lens or focusing lens) in order that occurrence of out-of-focus can be prevented during zooming operation (zoom magnification changing operation).
2. Description of the Related Art
Widely known as a control method for the video camera with autofocus function is a video signal detecting method wherein a high frequency band component contained in a brightness signal of a video signal from an image sensor is taken out, levels of high frequency band component within a predetermined imaging range are integrated to produce a focusing voltage, and the lens position is controlled such that the focusing voltage is maximized. On the other hand, an optics of the inner focus type in which focusing is effected by an image lens disposed at the final stage of a lens system is widely used in, for example, a public camcorder (video camera integral with VCR) because of advantages of light weight and compactness. In the lens configuration of the inner focus type, however, the image lens is disposed behind a variator lens, raising a problem that the in-focus position of the image lens changes as the zooming operation proceeds.
To cope with this problem, in the technique described in JP-A-4-21276, with the aim of making an image lens follow an in-focus point in response to zooming operation, the position of the image lens is corrected such that out-of-focus does not take place during zooming operation by paying out the image lens by a predetermined amount in a predetermined direction when a focusing voltage falls below a predetermined threshold value during the zooming operation.
In this conventional technique, however, the image lens is not driven before the focusing voltage during the zooming operation falls below the predetermined threshold value, with the result that depending on an object, an operation of repeating the out-of-focus state and the in-focus state tends to occur; and when out-of-focus has already taken place upon start of the zooming, it is difficult to draw the image lens into the in-focus state during the zooming operation.
Further, most zoom lens used in the aforementioned public camcorder are of the inner focus type which are particularly removed of a compensator lens. FIG. 2 shows, with parameters of several subject distances, characteristics (zoom tracing curves) representing the in-focus position of the image lens in relation to the magnification (the position of the variator lens) which are obtained with the compensator lens removed. For example, when an object at an infinitely far distance is imaged, the in-focus position of the image lens traces the lowermost zoom tracing curve of FIG. 2 as the zooming is operated from wide angle to telephoto. As the subject distance decreases, the zoom tracing curve shifts to those of subject distances B and A (distance A&lt;distance B). The uppermost zoom tracing curve of FIG. 2 then corresponds to a near distance. Since the zoom tracing curves are steeper than those of the conventional zoom lens with compensator, the image lens is required to follow the movement of the variator lens more rapidly. Further, because of speed-up of the zooming, it is very difficult to correct and drive the image lens smoothly without out-of-focus during the zooming operation.
U.S. Pat. No. 4,842,387 discloses an automatic focusing apparatus for video camera in which a variation signal of a high frequency band component contained in a video signal is generated when one optical element of a focusing lens system is vibrated slightly and focusing is effected on the basis of the polarity and magnitude of the variation signal.